A non-destructive inspection method using a magnetic phenomenon is useful to detect a surface defect of a structure, a near surface defect of the structure, or an internal defect of the structure. The non-destructive inspection method can be used to detect a defect of a large plant or structure used for nuclear power generation, a thermal power plant, chemical industry, etc.
A paramagnetic substance, such as an aluminum alloy or stainless steel, is used for airplanes or nuclear power generation equipment. A non-destructive inspection method using magneto-optical effects has attracted attention as a method of measuring damage to a paramagnetic substance. In particular, a Magneto-Optical eddy current Imager (MOI) has been developed by PRI R&D Co. to non-destructively detect the defects of an airplane. An image indicating a surface fatigue crack and erosion of a paramagnetic substance, a defect of a near surface of the paramagnetic substance, etc., can be obtained using the MOI.
FIG. 1A illustrates a surface fatigue crack in an aluminum alloy plate, which is a paramagnetic substance, wherein the surface fatigue crack is detected using an MOI. FIG. 1B illustrates a surface crack in austenite stainless steel formed of a mixture of ferromagnetic and paramagnetic substances, wherein the surface crack is detected using an MOI. In FIG. 1B, the surface crack and a martensite area are illustrated in the austenite stainless steel.
If an object that is to be measured is a paramagnetic substance as shown in FIG. 1A, surface cracks that are indistinguishable to the naked eye can be easily detected from the object using the MOI. However, if an object that is to be measured is a paramagnetic structure locally mixed with a ferromagnetic substance as shown in FIG. 1B, it is very difficult to detect surface cracks in the object using the MOI.
Also, if the MOI is used, it is difficult to quantitatively evaluate the leakage flux generated by the dispersion of an induced current around the surface cracks. In other words, since the MOI provides only image data regarding the surface cracks as shown in FIG. 1A or 1B, it is difficult to quantitatively evaluate actual shapes, sizes, or the like of the surface cracks. In addition, the detected results may be differently interpreted ac cording to the knowledge or skill of an inspector.